Fractional flow reserve (FFR) is physiological index that measures the functional severity of a coronary artery stenosis (i.e., a narrowing, and/or an occlusion of the artery that is usually due to atherosclerosis). FFR measures the severity of the stenosis by determining the maximal blood flow through the artery in the presence of the stenosis relative to the hypothetical level of blood flow through the artery, if the artery were healthy. FFR provides an indication of the likelihood that the stenosis is impeding and/or will impede oxygen delivery to the heart muscle (i.e., the likelihood that the stenosis is causing and/or will cause myocardial ischemia). Other luminal-flow-related indices that are used to measure conditions of the coronary circulatory system include instantaneous wave-free ratio (iFR), coronary flow reserve (CFR), index of microcirculatory resistance (IMR), microvascular resistance index (MVRI), TIMI myocardial perfusion grade (TMPG), relative fractional flow reserve (RFFR), and other related (e.g., other statistically correlated) indices.
FFR is typically utilized in coronary catheterizations, and is typically calculated by measuring pressure differences across a coronary artery stenosis. Assuming that there is single stenosis, the relationship between the pressure downstream of the stenosis and the pressure upstream of the stenosis approximates the relationship between the flow of blood in the currently-stenosed coronary artery and the normal flow of blood had the artery been healthy. Thus, measuring pressure differences across a coronary artery stenosis provides an approximation of the FFR.
Typically, FFR serves as a decision support tool for determining whether the stenosis should be treated, such as by means of inflating a balloon and implanting a stent.
FFR is defined as the ratio between stenotic flow QS and normal flow QN under hyperemic conditions: FFR=QS/QN 
Using the flow equation Q=ΔP/R, where Q is the flow (mL/min), ΔP is the pressure difference (mm Hg), and R is resistance (mmHg×min/mL), and the assumption that the venous pressure Pvein is negligible, the FFR can be expressed as the ratio between distal pressure Pd to proximal pressure Pa of a stenosis:FFR=(QS/QN)=((Pd−Pvein)/R)/((Pa−Pvein)/R)=Pd/Pa 
This pressure ratio can be written as follows:FFR=Pd/Pa=(Pa−ΔPs)/Pa where ΔPs is the pressure drop along the axis of the lumen along a segment of the lumen from upstream of the stenosis to downstream of the stenosis.
The FFR result is an absolute number between zero and one; an FFR of 0.50 means that a given stenosis causes a 50% drop in blood pressure. In other words, FFR expresses the maximal flow through a lumen in the presence of a stenosis compared to the maximal flow in the hypothetical absence of the stenosis.
Typically, FFR is measured in coronary vessels by means of inserting into such vessels a wire equipped with sensors. The device analyzes pressure and flow parameters from inside of the vessel. Such wires are currently being produced by Volcano Corp. (San Diego, Calif.) and by St. Jude Medical (St. Paul, Minn.).